“Synth sounds” covers a huge area.  For monophonic synthesizer-type leads, the obvious choice would be the Synthonizer algorithm.  But there are many synthesizer-like sounds that can be produced by the PitchFactor.

• Diatonic & QuadraVox for duo-monophonic or one-note ‘chord memory’ synth emulations.
• HarModulator for parallel ‘oscillators’, pitchbending and wild modulation effects.
• MicroPitch & H910 / H949 for ultra-fat supersaw synths, circuit-bent / lo-fi, ‘barberpole’, and spaceship FX.
• Octaver for deep analog bass synth and filter sweep sounds.
• Crystals for ADSR, reverse sweep, and ‘bell-tree’ effects.
• HarPeggiator for step sequencer, slicer, sample & hold, glitch and vintage game console chip-set FX.

If you have anything specific in mind, this forum is good for helping you to wring out the exact type of sounds that you’re after.